target-ppc: Extract aliases from definitions list

target-ppc: Extract aliases from definitions list

Move definitions that were 100% identical except for the name into a
list of aliases so that we don't register duplicate CPU types.
Drop the accompanying comments since they don't really add value.

We need to support recursive lookup due to code names referencing a
generic name referencing a specific model revision.

List aliases separately for -cpu ?.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>